It doesn't hurt that we traded away a couple like Frankie Montas and Justin Masterson, and converted some of the better ones left into relievers, like Barnes and Papelbon.
To say it's the scouting or development is probably not correct, since this goes back nearly two decades. And if you discount Lester and Buchholz, you have to go back to Aaron Sele in 1991 to find a SP who worked out for the Sox. That spans nearly 30 years and over half a dozen GMs and probably hundreds of scouts and minor league coaches. At this point, it is getting far too coincidental for my liking.
Maybe part of the reason is the Sox have had a habit of filling the rotation with veteran arms as they could afford them, and simply dealt their better pitching prospects away or converted them to bullpen arms...